There are eleven types of shift possible where a "shift" is available. This document describes them.

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The lower twelve bits of the instruction define the shift type.

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===Non shifts===

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There are two ''non-shift'' types - the ''immediate'' and a ''register'' reference.

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'''Immediate''' values are values that are directly encoded into the instruction, such as:

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ADD R0, R1, #123

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There is, however, a slight complication in that not all values can be represented. The immediate works by optionally shifting an eight bit value. Thus, any value up to &FF is valid. Likewise, it is possible to shift that around, so &F000000F is valid, as is &000AD000. However &00000123 is not (for &123 cannot be described by rotating an eight bit value).<br>

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In the case of requiring a larger value, you should ADD or SUB until you have the desired number, or LDR it from a data word.

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This is encoded into the instruction as:

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[[Image:Shift_immediate_constant.png|center]]

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'''Register''' values are references to a register, for example:

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ADD R0, R1, R2

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As a technical note, this is actually stored in memory as a Logical Shift Left by immediate with the shift being zero.

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This is encoded into the instruction as:

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[[Image:Shift_immediate_register.png|center]]

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===The barrel shifter===

===The barrel shifter===

The ARM processor incorporates a barrel shifter that can be used with the data processing instructions ([[ADC]], [[ADD]], [[AND]], [[BIC]], [[CMN]], [[CMP]], [[EOR]], [[MOV]], [[MVN]], [[ORR]], [[RSB]], [[SBC]], [[SUB]], [[TEQ]], [[TST]]). You can also use the barrel shifter to affect the index value in [[LDR]]/[[STR]] operations.

The ARM processor incorporates a barrel shifter that can be used with the data processing instructions ([[ADC]], [[ADD]], [[AND]], [[BIC]], [[CMN]], [[CMP]], [[EOR]], [[MOV]], [[MVN]], [[ORR]], [[RSB]], [[SBC]], [[SUB]], [[TEQ]], [[TST]]). You can also use the barrel shifter to affect the index value in [[LDR]]/[[STR]] operations.

The shift amount can be from one to thirty two, and the carry-out is the value of the last bit shifted out.

The shift amount can be from one to thirty two, and the carry-out is the value of the last bit shifted out.

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This shift is encoded for immediate shifts as:

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[[Image:Shift_LSR_immediate.png|center]]

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and for register shifts as:

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[[Image:Shift_LSR_register.png|center]]

===Arithmetic Shift Right===

===Arithmetic Shift Right===

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This is similar to LSR, with the exception that the high bits are filled with the value of bit 31 of the register being shifted (R''x''), in order to preserve the sign in 2's complement maths. It is otherwise the same as <code>register = value >> shift</code>.

This is similar to LSR, with the exception that the high bits are filled with the value of bit 31 of the register being shifted (R''x''), in order to preserve the sign in 2's complement maths. It is otherwise the same as <code>register = value >> shift</code>.

[[Image:Diagram ASR.png|center|Diagram of ASR]]

[[Image:Diagram ASR.png|center|Diagram of ASR]]

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This shift is encoded for immediate shifts as:

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[[Image:Shift_ASR_immediate.png|center]]

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and for register shifts as:

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[[Image:Shift_ASR_register.png|center]]

===Rotate Right===

===Rotate Right===

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[[Image:Diagram ROR.png|center|Diagram of ROR]]

[[Image:Diagram ROR.png|center|Diagram of ROR]]

If it was possible to specify the amount to be shifted as 32, the result would be that all the bits would be rotated by 32 places, right back to their original positions.

If it was possible to specify the amount to be shifted as 32, the result would be that all the bits would be rotated by 32 places, right back to their original positions.

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It is not possible to ROR #0, as that encoding is used to signify RRX.

The carry-out is the value of the last bit shifted.

The carry-out is the value of the last bit shifted.

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This shift is encoded for immediate shifts as:

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[[Image:Shift_ROR_immediate.png|center]]

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and for register shifts as:

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[[Image:Shift_ROR_register.png|center]]

===Rotate Right with Extend===

===Rotate Right with Extend===

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This is a <code>ROR #1</code> operation, which rotates ''one'' place to the right - the difference is that the processor's Carry flag is used to provide a 33 bit quantity to be shifted.

This is a <code>ROR #1</code> operation, which rotates ''one'' place to the right - the difference is that the processor's Carry flag is used to provide a 33 bit quantity to be shifted.

[[Image:Diagram RRX.png|center|Diagram of RRX]]

[[Image:Diagram RRX.png|center|Diagram of RRX]]

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This shift is encoded for immediate shifts as:

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[[Image:Shift_RRX.png|center]]

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It is encoded into the space that would be used for ROR, #0.

Revision as of 18:45, 31 August 2011

There are eleven types of shift possible where a "shift" is available. This document describes them.

Contents

Non shifts

There are two non-shift types - the immediate and a register reference.

Immediate values are values that are directly encoded into the instruction, such as:

ADD R0, R1, #123

There is, however, a slight complication in that not all values can be represented. The immediate works by optionally shifting an eight bit value. Thus, any value up to &FF is valid. Likewise, it is possible to shift that around, so &F000000F is valid, as is &000AD000. However &00000123 is not (for &123 cannot be described by rotating an eight bit value).
In the case of requiring a larger value, you should ADD or SUB until you have the desired number, or LDR it from a data word.

This is encoded into the instruction as:

Register values are references to a register, for example:

ADD R0, R1, R2

As a technical note, this is actually stored in memory as a Logical Shift Left by immediate with the shift being zero.

This is encoded into the instruction as:

The barrel shifter

The ARM processor incorporates a barrel shifter that can be used with the data processing instructions (ADC, ADD, AND, BIC, CMN, CMP, EOR, MOV, MVN, ORR, RSB, SBC, SUB, TEQ, TST). You can also use the barrel shifter to affect the index value in LDR/STR operations.

There are six mnemonics for the different shift types:

LSL Logical Shift Left

ASL Arithmetic Shift Left

LSR Logical Shift Right

ASR Arithmetic Shift Right

ROR Rotate Right

RRX Rotate Right with Extend

ASL and LSL are the same, and may be freely interchanged.

You can specify a shift with an immediate value, or by a register which holds the value to be shifted by.

Logical or Arithmetic Shift Left

Rx, LSL #nor
Rx, ASL #nor
Rx, LSL Rnor
Rx, ASL Rn

The contents of Rx will be taken and shifted to a more significant position by the amount specified by n or in the register Rn. The shift amount can be from zero to thirty one. The least significant bits introduced are zeroes.

The high bits shifted off to the left are discarded, except for the notional bit thirty three (i.e., the last bit to be shifted off) which becomes the value of the carry flag on exit from the barrel shifter.

Logical Shift Right

This does the notional opposite of shifting left. Everything is shifted to the right, to a less significant position. It is the same as register = value >>> shift.

The shift amount can be from one to thirty two, and the carry-out is the value of the last bit shifted out.

This shift is encoded for immediate shifts as:

and for register shifts as:

Arithmetic Shift Right

Rx, ASR #nor
Rx, ASR Rn

This is similar to LSR, with the exception that the high bits are filled with the value of bit 31 of the register being shifted (Rx), in order to preserve the sign in 2's complement maths. It is otherwise the same as register = value >> shift.

This shift is encoded for immediate shifts as:

and for register shifts as:

Rotate Right

Rx, ROR #nor
Rx, ROR Rn

Rotate Right is similar to a Logical Shift Right, except the bits which would normally be shifted off the right are replaced on the left, thus the bits 'rotate'.

If it was possible to specify the amount to be shifted as 32, the result would be that all the bits would be rotated by 32 places, right back to their original positions.

It is not possible to ROR #0, as that encoding is used to signify RRX.

The carry-out is the value of the last bit shifted.

This shift is encoded for immediate shifts as:

and for register shifts as:

Rotate Right with Extend

Rx, RRX

This is a ROR #1 operation, which rotates one place to the right - the difference is that the processor's Carry flag is used to provide a 33 bit quantity to be shifted.